3 Answers
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You can send emails only to people who have usernames claimed. Facebook doesn't allow sending emails to IDs.

Below's my original (wrong) answer.

Yes, the email is username@facebook.com. Your friends should receive them as long as you send them from an email associated with your Facebook account because the default setting is to accept messages from at least friends.

You likely didn't receive your test message because your setting is currently something restrictive. Emails to username@facebook.com are controlled by the same setting as all messages sent on Facebook, which is "Who can send you Facebook messages?" under How You Connect in Privacy Settings.

Sending an email to a username is only effective if that particular name had the unlikely chance of being unique. If there are several identical user names facebook assigns a number following the username. To see the email address of a user, click on the ABOUT button on their home page.